The present invention relates to a continuous casting apparatus of twin-drum type in which a molten steel is held in a pool defined by a pair of rolls or drums and a pair of side dams, the drums being rotatable in counter directions so that the molten steel is continuously extracted downward through the nip between the two drums, whereby a thin steel sheet is formed continuously.
Japanese Patent Laid Open No. 187244/1983 disclosed an example of the twin-drum type continuous casting apparatus having two drums which cooperate in defining therebetween a pool of molten steel. The molten steel in the pool is partially solidified to form solidification shells contacting these drums. As the drums rotate in counter directions so as to pinch and pull the molten steel downward, these solidification shells are pressure-bonded to each other when they pass through a gap between the two drums, whereby a steel sheet is formed continuously.
Referring to FIGS. 1 and 2, during the continuous casting of sheet 5 of thickness t with this casting apparatus, the solidification takes place earlier in the edge regions of the sheet than in the center of the same. In addition, when the semi-solidified steel passes through the smallest gap 4 between the drums 1, 1', at which gap 4 the pressure-bonding is finished, the solidification shells 3, 3' are pressed to each other to produce reactional pressure P, which acts to urge both drums away from each other as indicated by arrows in FIG. 1. At the same time, a pressure P3 is produced also in the lateral direction in the pressure-bonded portion of gap 4, i.e., in the breadthwise direction of the sheet, as illustrated in FIG. 2. This lateral pressure P3 acts to urge both side dams 2, 2' away from each other at portions of these dams confronting the pressure-bonded portion of gap 4 between the two drums. This lateral pressure becomes greater as the solidification proceeds, as explained in the above-mentioned Japanese Patent Laid-Open No. 187244/1983.
Generally, the molten steel temperature for casting steel sheet is as high as 1550.degree. C. or so. The side dams 2, 2', therefore, are made of refractory bricks so that they may withstand this high temperature.
On the other hand, the lateral pressure P3 generated in the pressure-bonded portion between the drums which pressure-bonds the solidification shells is as high as about 200 kg/cm.sup.2, because the solidification shells, which have been cooled down to 1350.degree. to 1400.degree. C., exhibit a deformation resistance which is substantially the same as that of hot steel.
Unfortunately, the side dams 2, 2' made of refractory bricks exhibit an extremely low strength at high temperature, and the dams are rapidly worn down due to abrasion by the pressure P3 shown in FIG. 2 as the thin sheet 5 is pulled downwardly. The wear of the side dams 2, 2' would be suppressed if the force by which the side dams are pressed is lowered. Such a reduced force, however, will allow the side dams 2, 2' to be displaced outwardly in the breadthwise direction of the sheet to thereby cause a side gap between the side dams 2, 2' and corresponding axial ends of the drums 1, 1' which gaps cause the molten steel to escape therethrough resulting in the formation of cast burr in the cast product, thus making it difficult to put the twin-drum type continuous casting apparatus into practical use.
In order to obviate this problem, the aforementioned Japanese Patent Laid-Open No. 187244/1983 proposes to use a material having a small heat conductivity in the portions of the drums corresponding to the breadthwise ends of the sheet. According to this proposal, the thickness of the solidification shell is small in the regions contacting the portions of the drums of the smaller heat conductivity so that the pressure caused when pressure-bonding is effected becomes smaller in such regions than in the breadthwise central portion, thus contributing to prolongation of the service life of the refractory side dams.
From a practical point of view, however, it is not preferred to construct the drum from two different kinks of materials having different physical properties, because such a drum is complicated construction and because a gap is apt to occur in a boundary between two kinds of materials into which gap molten steel is apt to leak to thereby make the casting impossible.
In most cases, the side dams are intended to be forced onto the axial end surfaces of the drums so as to form the pool of the molten steel, as explained in Japanese Patent Laid-Open No. 218358/1983. Such side dams are made from a refractory material, whereas the drums are made of a metal having a superior cooling property. Therefore, the side dams made of refractory material exhibit a higher temperature than the drums, so that the side dams are fragile and worn down rapdily.
Generally, when the casting speed is 30 m/min, the refractory material constituting the side dams is worn down in quite a short time, say about 1 minute. In consequence, a clearance is formed between each end of the drums and the opposing side dam. Thus, the apparatus cannot be used satisfactorily for the purpose of continuous casting for a long sheet.
On the other hand, Japanese Patent Laid-Open No. 38640/1983 discloses a twin-belt type continuous casting apparatus which employs stationary side plates each of which is constituted by a tapered refractory portion projected into the molten steel and a quenching metallic portion arranged in conformity with the breadth of the sheet, and a thickness adjusting roll which is intended for supporting both the solidified shells and the static pressure of the molten steel. Since this roll is not intended for the rolling (or pressure-bonding), no lateral spreading is caused by the rolling, so that the value of projection of the refractory material may be as small as several millimeters which correspond to the amount which may be lost by melting or exfoliation. It is also considered that the quenching metal plate can function satisfactorily if it is disposed in the vicinity of the thickness adjusting roll or downstream therefrom.
Unlike the twin-belt type apparatus, the twin-drum type continuous casting apparatus for directly casting a thin sheet of several millimeters essentially requires the rolling or pressure-bonding of a material immediately after the formation of the solidification shells, in order to obtain high quality of the cast product not only in the surface regions but also in the core portion of the product. It is, therefore, necessary to find a suitable construction and arrangement of the side dams. In other words, a suitable mechanism is essentially required for preventing the clearance from being caused between each of axial ends of the drums and each of opposing side dams, while allowing the cast material to be spread in the breadthwise direction.
Japanese Patent Laid-Open No. 21524/1974 discloses a twin-roll type casting apparatus in which the speed of the rolls is increased when the breadthwise spreading of the cast material during pressure-bonding of the solidification shells has increased a predetermined amount. On the other hand, Japanese Patent Laid-Open No. 21525/1984 discloses an apparatus in which side dams are moved upward in accordance with the amount of lateral spreading of the material during pressure-bonding of the solidification shells. In these known apparatus, however, the side dams which cooperate with the rolls or drums in defining the pool for the molten steel are made solely of a refractory material, and are inevitably damaged or worn as the material is largely spread laterally as a result of pressure-bonding of the solidification shells, which is necessary for attaining a high quality in the core part of the cast sheet. Thus, the requirement for the protection of the side dams and the requirement for the high quality of the core portion of the cast steel are incompatible with each other.